Information terminal with positioning function, positioning system, method of positioning, storage medium, and computer program product

ABSTRACT

From a signal transmitted from a satellite, are extracted a position where a terrestrial station side system exists and a position of the satellite. A distance between the terrestrial station side system and the satellite, based on a difference of those positions. Then, a sum of the distance between the terrestrial station side system and the satellite and a distance between the satellite and a mobile station side system, based on a difference between the reception time when a mobile station side communication antenna receives the signal and the transmission time. Further, the distance between the mobile station side system and the satellite is obtained by subtracting the distance between the terrestrial station side system and the satellite from the obtained sum. And, based on the distance between the mobile station side system and the satellite, the position where the mobile station side system exists is obtained.

[0001] This application is based on Japanese Patent Application No.2000-268372 filed in Japan, the contents of which are incorporatedhereinto by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a positioning technique forusing a signal transmitted from an artificial satellite to measure aposition of an information terminal provided to a mobile station sidesystem.

[0004] 2. Related Art Statement

[0005] Japanese Patent Laid-Open No. 11-34996 describes a positioningtechnique using a non-geostationary general-purpose satellite that moveson a long elliptical orbit and can provide services without beingaffected by landforms and shadows by building arrangement within aspecific service area (for example, the area of a certain country, suchas the whole of Japan including isolated islands and the range ofterritorial waters) (hereinafter, referred to as a quasi-zenithalsatellite), wherein the satellite is provided with a communicationsystem, goes around on the elliptical orbit in a 24-hour cycle, and isused for the positioning in the range of orbit inclination of more thanor equal to 37 degrees and less than or equal to 44 degrees and in therange of eccentricity of more than or equal to 0.24 and less than orequal to 0.35 (hereinafter, referred to as an “HEO satellite”).

[0006] A method of detecting a position by a navigation apparatusutilizing a non-geostationary general-purpose satellite is described inJapanese Unexamined Patent Laid-Open No. 10-48310. According to thedetection method of 10-48310, a terrestrial station concerned transmitsan RF signal to a user station through a forward link (satellitecommunication), and conversely, the user station replies to theterrestrial station through a return link (satellite communication).Then, based on the round-trip propagation time of this communication,the range between the terrestrial station and the user terminal (the sumof the distance between the terrestrial station and thenon-geostationary general-purpose satellite and the distance between thegeostationary general-purpose satellite and the user terminal) iscalculated. Further, based on thus-calculated range and a known rangebetween the geostationary general-purpose satellite and the terrestrialstation, the user station performs positioning calculation of the rangebetween the satellite and the user terminal. Further, this positioningcalculation is performed based on the solutions by respective Dopplereffects generated between the terrestrial station concerned and thesatellite and between the satellite and the user station, and based onthe above-calculated range.

[0007] Further, Japanese patent Laid-Open No. 8-331033 describespositioning utilizing elliptical orbit communication satellites. Inparticular, its paragraph 0081 describes calculation of distance betweena communication satellite and a mobile station by obtaining a differencebetween a radio wave propagation time of a radio channel making a roundtrip between a satellite communication fixed station and a mobilestation through the communication satellite and a radio wave propagationtime of a radio channel making a round trip between the satellitecommunication fixed station and the communication satellite, and bymultiplying the obtained difference by the radio wave propagationvelocity. Further, the paragraph 0084 of the same document describesthat received field strengths of radio channels between a satellitecommunication fixed station or a ground communication terrestrialstation and a mobile station are obtained, and a radio channel havingthe largest received field strength among those radio channels isselected, and the selected channel is used to connect a communicationline.

[0008] In the positioning method described in 11-34996, an HEOsatellite, which is quasi-zenithal, is used for positioning. However, itdoes not describe what positioning method is favorable. In particular,it does not consider making Geometrical Dilution Of Precision (GDOP),which expresses the positioning precision, less than or equal to 10(less than or equal to 11 or 9, when an error of 10% for the GDOP value10 is included) at all.

[0009] Further, the positioning method described in 10-48310 utilizesgeneral-purpose non-geostationary satellites. However, communication isperformed bilaterally (forward link and return link) between aterrestrial station concerned and a user station through a satellite,and the terrestrial station concerned performs positioning calculation.Accordingly, the terrestrial station must perform transmissions andreceptions four times, in order to perform positioning calculation.Further, in order that the user station itself can know its position, itmust perform transmission and reception, further. In other words,reduction of positioning time including times for transmissions andreceptions is not taken into consideration.

[0010] Considering the environment of the present communication systemthat the communication capacity is 2 MBPS or less while there aremillions of users of information terminals having a positioning functionsuch as car navigation terminals, it is difficult that the mentionedpositioning method ensures the real time property of positioning in, forexample, a car navigation system. Further, this conventional techniquealso does not consider how GDOP, which expresses a positioningprecision, can be made not more than 10, at all.

[0011] Further, as described above, the positioning method of 8-331033obtains a distance between a satellite communication fixed station and acommunication satellite based on a radio wave propagation time of aradio channel that makes a round trip between the satellitecommunication fixed station and the communication satellite. Thus, itdoes not consider a delay error of radio wave propagation generated inthe ionosphere when a radio channel makes a round trip between thesatellite communication fixed station and the communication satellite,and an error of radio wave propagation time generated between a clockprovided in the satellite communication fixed station and a clockprovided in the communication satellite. In other words, it does notconsider high precision positioning that suppresses effects of a radiochannel making a round trip between the satellite communication fixedstation and the communication satellite on radio wave propagation. Inparticular, it does not consider making GDOP not more than 10, at all.

[0012] Further, as described above, the conventional positioning methodsdo not considere making GDOP (Geometrical Dilution Of Precision) notmore than 10, though it is necessary to make GDOP less than or equal to10 as described in “GPS”, p. 135 (published by Japanese Association ofSurveyors on Nov. 5, 1989). In other words, the conventional methods donot consider what operating conditions can make GDOP less than or equalto 10 when a quasi-zenithal satellite (in particular, an HEO satellite)is used for positioning.

[0013] Further, when there are a plurality of terrestrial station sidesystems, a terrestrial station through which connection with atelecommunication business is established is not selected inconsideration of reduction of communication time and communication costbetween the telecommunication business and the terrestrial station sidesystem.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is to provide an informationterminal and positioning system having a positioning function thatensures a high positioning precision in a short time.

[0015] Another object of the present invention is to make GDOP, which isan index of positioning precision, less than or equal to 10, inparticular, in positioning utilizing quasi-zenithal satellites(particularly, HEO satellites).

[0016] Further, another object of the present invention is to reduce acommunication time and communication cost in providing Internetconnection service through satellites.

[0017] The present invention provides a positioning system providedwith:

[0018] (1) a terrestrial station side system comprising: a terrestrialstation side satellite communication antenna for transmitting a signalto a satellite; and a terrestrial station side communication apparatusfor transmitting a signal to the terrestrial station side satellitecommunication antenna;

[0019] (2) the satellite having a satellite side satellite communicationantenna for transmission and reception to and from the ground; and

[0020] (3) a mobile station side system comprising: a mobile stationside satellite communication antenna for receiving the signal from thesatellite; and an information terminal for measuring a position wherethe mobile station side system exists, based on the signal receivedthrough the mobile station side satellite communication antenna.

[0021] Further, the present invention provides: a positioning methodused for the positioning system of the present invention; an informationterminal (including a portable terminal) that can perform positioningusing the mentioned method; a computer readable storage medium thatstores a computer program for realizing the mentioned positioningmethod; and a computer program product having computer readable programcode means for realizing the mentioned positioning method.

[0022] The positioning method of the present invention comprises stepsof:

[0023] extracting a distance between a position where a terrestrialstation side system exists and a satellite, from a signal transmittedfrom the satellite;

[0024] extracting a transmission time from the signal transmitted fromthe satellite;

[0025] obtaining a sum of the distance (a) between the terrestrialstation side system and the satellite and a distance (b) between thesatellite and a mobile station side system;

[0026] obtaining the distance (b) between the mobile station side systemand the satellite by subtracting the distance (a) between theterrestrial station side system and the satellite from the sum (a+b);and

[0027] obtaining a position where the mobile station side system exists,based on the obtained distance (b) between the terrestrial station sidesystem and the satellite.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] These and other features, objects and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings wherein:

[0029]FIG. 1 is a diagram showing a configuration of a positioningsystem according to one embodiment of the present invention;

[0030]FIG. 2 is a block diagram showing a terrestrial station sidesystem;

[0031]FIG. 3 is a block diagram showing an HEO satellite;

[0032]FIG. 4 is a block diagram showing a mobile station side system;

[0033]FIG. 5 is a flowchart showing processing included in theterrestrial station side system;

[0034]FIG. 6 shows contents of a navigation message;

[0035]FIG. 7 is a block diagram showing a GPS information generatingpart;

[0036]FIG. 8 is a flowchart showing processing performed by a mobilestation;

[0037]FIG. 9 shows measured data of GDOP;

[0038]FIG. 10 shows position data of an HEO satellite;

[0039]FIG. 11 is a diagram showing an entire system configurationaccording to another embodiment;

[0040]FIG. 12 is a diagram showing a configuration of the terrestrialstation side system of FIG. 11;

[0041]FIG. 13 is a diagram showing a configuration of the referencestation side system of FIG. 11;

[0042]FIG. 14 is a diagram showing a configuration of the mobile stationside system of FIG. 11;

[0043]FIG. 15 is a block diagram showing the GPS information generatingpart of FIG. 12;

[0044]FIG. 16 shows contents of DGPS information used in the system ofFIG. 11;

[0045]FIG. 17 is a diagram showing an entire system configurationaccording to another embodiment; and

[0046]FIG. 18 is a flowchart showing processing performed by the mobilestation side system of FIG. 17.

[0047] In each drawing, the reference numeral 101 refers to an HEOsatellite; 102 to a terrestrial station side system, 103 to a mobilestation side system, 105 to a terrestrial station side informationterminal, 121 to a GPS information generating part, 122 to a clockinformation generating part, 123 to a satellite position deciding part,124 to a terrestrial station control part, 125 to a data selection andtransmission part, 126 to a terrestrial station position storing part,127 to a data separation part, 129 to a terrestrial station sidesatellite communication antenna, 136 to a mobile station side satellitecommunication antenna, 131 to a terminal control part, 132 to a positioncalculation part, 133 to a data separation part, 134 to a map datastoring part, 135 to a clock information generating part, 137 to aterrestrial station side position information storing part, 151 to atransmission timing calculation part, and 152 to a decode informationstoring part.

DETAILED DESCRIPTION OF THE INVENTION

[0048] As one mode of the present invention, can be considered afollowing mode.

[0049] Namely, a position of a terrestrial station side system and aposition of a satellite are extracted from a signal transmitted from thesatellite, and a distance between the terrestrial station side systemand the satellite is obtained from a difference between those positions.Here, when the signal transmitted from the satellite includes thedistance between the terrestrial station side system and the satellite,the distance is obtained directly. Next, a sum of the distance betweenthe terrestrial station side system and the satellite and a distancebetween the satellite and a mobile station side system is obtained basedon a difference between a reception time when a mobile station sidecommunication antenna receives the signal and a transmission time ofthat signal. Then, the distance between the mobile station side systemand the satellite is obtained by subtracting the distance between theterrestrial station side system and the satellite from theabove-mentioned sum, and a position where the mobile station side systemexists is obtained based on the distance between the mobile station sidesystem and the satellite.

[0050] According to this mode, the distance between the terrestrialstation side system and the satellite (or, the positions of theterrestrial station side system and the satellite, which can specify thedistance between them) is transmitted in advance from the terrestrialstation side system, and the information terminal receives that distance(or, those positions) through the satellite.

[0051] Thus, positioning processing can be performed in the informationterminal only, and it is possible to provide the information terminalhaving a real-time and highly precise positioning function.

[0052] Further, another mode of the present invention provides apositioning system comprising:

[0053] a terrestrial station side system that in turn comprises: aterrestrial station side satellite communication antenna fortransmitting signals to satellites; and a communication apparatus fortransmitting the signal to the terrestrial station side satellitecommunication antenna;

[0054] the satellites each having a satellite side satellitecommunication antenna for transmission and reception to and from theground; and

[0055] a mobile station side system that comprises: a mobile stationside satellite communication antenna for receiving the signals from thesatellites; and an information terminal for measuring a position wherethe mobile station side system exists, based on the signals receivedthrough the mobile station side satellite communication antenna;wherein:

[0056] the above-mentioned satellites include three or morequasi-zenithal satellites; and

[0057] the information terminal measures the position where the mobilestation side system exists, based on signals received from those threeor more quasi-zenithal satellite.

[0058] According to this mode, it is possible to make GDOP less than orequal to 11, and to ensure sufficient precision for positioning. Here,when five or more quasi-zenithal satellites are provided, the value ofGDOP can be less than or equal to 10 even if its error is estimated as10%, and sufficient precision for positioning can be ensured.

[0059] Further, when a constituent apparatus of a positioning system(such as an information terminal of a mobile station side system)selects a terrestrial station side system, considering communicationenvironment (in particular, communication capacity) of a communicationline between a telecommunication business, which connects acommunication line for Internet or the like, and a terrestrial stationside system, to connect a line with a communication apparatus of thetelecommunication business, it is possible to provide comfortablecommunication environment such as reduced communication time to a userof the information terminal.

[0060] According to the present invention, it is possible to provide aninformation terminal and positioning system having a positioningfunction that ensures high precision in short time.

[0061] Further, according to the present invention, it is possible toprovide an information terminal and positioning system having apositioning function in which GDOP as an index of positioning precisionis less than or equal to 10.

THE PREFERRED EMBODIMENTS

[0062]FIG. 1 shows a positioning system according to one embodiment ofthe present invention.

[0063] The positioning system of the present embodiment comprises: fiveof the above-mentioned HEO satellites 101 (one is omitted in FIG. 1); aterrestrial station side system 102 including terrestrial station sidesatellite communication antennas 129 and a terrestrial station sidecommunication apparatus 104; and a mobile station side system 103including a mobile station side satellite communication antenna 136 andan information terminal 105.

[0064] In the terrestrial station side system 102, in order to transmita radio wave (signal) to each HEO satellite 101, the terrestrial stationside communication apparatus 104 controls the terrestrial station sidesatellite communication antennas 129, and performs generation of anavigation message, generation of a GPS signal by modulation of thenavigation message, generation of GPS information 141 by modulation ofthe GPS signal, transmission of the GPS information to the terrestrialstation side antennas 129, transmission of a signal including the GPSinformation 141 toward the space in which each HEO satellite 101 exists,and the like.

[0065] Each HEO satellite 101 receives the GPS information 141transmitted from the terrestrial station side system 102, amplifies theGPS information 141, and thereafter transmits it towards the ground.

[0066] The mobile station side system 103 receives the GPS information141 from each HEO satellite 101 through the receiving antenna 136,decodes the received GPS information 141 by means of the informationterminal 105 to obtain the GPS signal and the navigation message, andcalculates the position of the mobile station side system using theobtained GPS signal and navigation message.

[0067] Here, the terrestrial station side system 103 is used as anavigation system when it is mounted on a moving object such as anautomobile moving in the environment having up-and-down geographicfeatures, and the information terminal 105 constituting the terrestrialstation side system is used as a navigation terminal.

[0068] Further, in the above description, the position of theterrestrial station side system 103 is measured. However, theinformation terminal 105 itself or the receiving antenna 136 itself hasthe almost same position as the terrestrial station side system 103, andtherefore, measuring the terrestrial station side system 103 includesthe case of measuring the information terminal 105 or the receivingantenna 136 also.

[0069] Further, in this specification, the GPS information 141 is asignal including a navigation message 141 and a GPS signal obtained bycoding a navigation message with a pseudo-spread code of each satellite.

[0070] Next, the embodiment of the positioning system of the presentinvention will be described in more detail.

[0071] (1) Terrestrial Station

[0072] The terrestrial station side system 102 comprises the terrestrialstation side communication apparatus 104, the terrestrial station sidesatellite communication antennas 129 as satellite communicationantennas, and the receiving antenna 128.

[0073] The configuration of the terrestrial station side communicationapparatus 104 included in this terrestrial station side system 102 willbe described referring to FIG. 2.

[0074] The terrestrial station side communication apparatus 104comprises a GPS information generating part 121, a clock informationgenerating part 122, a satellite position deciding part 123, aterrestrial station side control part 124, a data selection andtransmission part 125, a terrestrial station position storing part 126,a data separation part 127, a transmission timing calculation part 151,and a decode information storing part 152.

[0075] Next, functions of each component will be described.

[0076] The terrestrial station side control part 124 controls eachcomponent so that GPS information 141 (a signal obtained bysuperimposing a GPS signal and a navigation message) is periodicallytransmitted to each HEO satellite 104.

[0077] The satellite position deciding part 123 observes the position ofeach HEO satellite 101 (navigation position), to obtain the positiondata of the observed HEO satellite. This observation usestrigonometrical survey using laser from the ground.

[0078] The clock information generating part 122 utilizes a highprecision atomic clock such as a cesium clock or a rubidium clock, togenerate clock information for expressing the present time.

[0079] The terrestrial station position storing part 126 storesterrestrial station position information expressing the position of theterrestrial station side system 102 itself.

[0080] The GPS information generating part 121 adds the satelliteposition information, which is decided by the satellite positiondeciding part 123, and the terrestrial station position information,which expresses the position of the terrestrial station side system andis stored by the terrestrial station position storing part 126, to theconventional navigation message including orbit information and acalendar, to generate a new navigation message as shown in FIG. 6 (thisnew navigation message is referred to as “navigation message” in thespecification of the present invention). Then, by performing spreadspectrum modulation (hereinafter, referred to simply as “modulation”)with a pseudo-noise code (PN code, C/A code, and P code) that is decidedin advance for each HEO satellite, the GPS signal is superimposed to thenavigation message. Further, by superimposing the time (transmissiontime) generated by the above-mentioned clock information generatingpart, GPS information 141 is generated. (For example, in the case of theband of 25 M, the GPS information is generated using a pseudo-noise codeof 1023 (1024-1) bits decided for each satellite as a spread code(spread spectrum modulation) with a chip rate of 12.5 Mbps, similarly toNAVSTAR).

[0081] The data selection and transmission part 125 tracks each of theplurality of HEO satellites 101, controls each terrestrial station sidesatellite communication antenna 129 prepared for each HEO satellite 101to turn in the direction that makes transmission toward thecorresponding HEO satellite possible, and transmits the GPS information141 toward the HEO satellites through the respective terrestrial stationside satellite communication antennas 129. Further, the GPS information141 to transmit is obtained (selected) and used for each HEO satellite101 of the transmission destination, obtaining it from the GPSinformation generating part 121.

[0082] The decode information storing part 152 stores decode informationused for decoding GPS information 141 from a signal received through thereceiving antenna 128.

[0083] The data separation part 127 obtains the decode information fromthe decode information storing part 152, and separates the signalreceived through the receiving antenna 128 into the GPS information 141and the other signals. Further, by despreading the GPS information 141,is obtained a GPS signal. And, by further despreading the GPS signal, isobtained a navigation message.

[0084] The transmission timing calculation part 151 holds presupposedtransmission timing (time) for transmission to each HEO satellite 101.Further, from the GPS information 141 separated by the data separationpart 127, the transmission timing calculation part 151 obtains thetransmission time when the terrestrial station side system 102transmitted the GPS information 141, and obtains the reception time fromthe clock information obtained from the clock information storing part122. From the difference between those times, is obtained the timeelapsed from the actual transmission of the GPS information 141 throughthe terrestrial station side communication antenna 129 of theterrestrial station side system 102 to the arrival to the ground througheach HEO satellite 101. Next, based on this elapsed time, thetransmission time to each HEO satellite 101 and the presupposedtransmission time are compared. An average of such differences isobtained at intervals of a constant period (for example, at intervals ofone hour), and a transmission time is corrected by increasing ordecreasing the presupposed transmission time by this average, to obtaina transmission time that takes the ionospheric error into consideration.

[0085] Next, operation performed by the terrestrial station sideapparatus 104 controlled by the terrestrial station control part 124included in the terrestrial station side system 102 will be describedreferring to FIG. 5.

[0086] The terrestrial station control part 124 instructs the dataselection and transmission part 125 to select an HEO satellite 101 towhich GPS information 141 is transmitted, and to transmit the GPSinformation 141 to a terrestrial station side satellite communicationantenna 129. Receiving the instruction, the data selection andtransmission part 125 instructs the GPS information generating part 121to generate GPS information 141. The GPS information generating part 121obtains the satellite position information expressing the position ofeach HEO satellite from the satellite position deciding part 123,obtains the present time from the clock information generating part 122,and obtains the terrestrial station position information expressing theposition of the terrestrial station side system from the terrestrialstation position storing part 126 (Process 501).

[0087] The GPS information generating part 121 generates a navigationmessage that includes the obtained terrestrial station positioninformation and the satellite position information from the satelliteposition deciding part (Process 502).

[0088] Further, the GPS information generating part 121 performsspectrum spreading on the generated message to superimpose the obtainedclock information to the navigation message, and thus, to generate a GPSsignal (Process 503).

[0089] Then, the GPS information generating part 121 performs furtherspectrum spreading on the generated GPS signal to generate GPSinformation 141 added with an identification number for each HEOsatellite 101 as a destination of transmission, and then, transmits thegenerated GPS information 141 to the data selection and transmissionpart 125 (Process 504).

[0090] Next, the data selection and transmission part 125 selects theGPS information 141 and the HEO satellite to which the GPS information141 is transmitted, based on the identification number added to the GPSinformation 141, and further modulates the GPS information 141 with apredetermined frequency (Process 505).

[0091] Further, the data selection and transmission part 125 tracks eachHEO satellite 101 furthermore, and controls the terrestrial station sidesatellite antennas 129 to turns to suitable directions so that it ispossible to transmit each GPS information 141 to the HEO satellite 101indicated by the identification number added to the GPS information 141(Process 506).

[0092] Further, the data selection and transmission part 125 makes eachcontrolled terrestrial station side satellite communication antenna 129transmit the GPS information 141 for the corresponding HEO satellite ata transmission timing obtained by the transmission timing calculationpart 151 (Process 507).

[0093] The navigation message generated in Process 502 according to thepresent invention is shown in FIG. 6.

[0094] As shown in the figure, the navigation message includes both theterrestrial station position information expressing the position of theterrestrial station and the position information of the HEO satellite101, or distance information expressing a distance between theterrestrial station and the HEO satellite 101, in addition to theconventional navigation message. Since the GPS information 141 includesthe time information of the transmission timing (transmission timeinformation), and an information terminal uses those pieces ofinformation in the positioning calculation, the real time property andhigh precision positioning is realized.

[0095] Next, detailed operation of the above-mentioned GPS informationgenerating part 121 will be described referring to FIG. 7.

[0096] The GPS information generating part 121 comprises: a carriergenerating part 704 for generating a carrier P(t) in accordance with aclock from the clock information generating part 122; a spread codegenerating part 703 for sequentially generating a GPS signal (spreadcode (PN code) C(t)) characteristic to each satellite, synchronouslywith the clock from the clock information generating part 122; anavigation message generating part 702 for repeatedly generating thenavigation message shown in FIG. 6 synchronously with the clockinformation generating part 122; and a delay part 705 for calculating adelay time based on the distance between the terrestrial station 102 andeach HEO satellite 101 and for generating a delay of the GPS signalcorresponding to a difference from the periodic difference of the PNcode to approximate the above-mentioned delay time as shown in FIG. 8.

[0097] The GPS information generating part 121 superimposes the GPSsignal to the navigation message by using the spread code C(t) to spreada data sequence D(t) of the navigation message generated by thenavigation message generating part 702.

[0098] Further, the carrier generating part 704 modulates the result ofthe superimposition to generate GPS information 141. At that time, thedelay part 705 generates a delay to bring a delay of one period of thePN code, which can cancel the communication time error generated betweenthe HEO satellite and the terrestrial station.

[0099] Thus, the GPS information 141 includes two signals, namely, thenavigation message D(t) and the GPS signal obtained by superimposing thenavigation message with the spread code C(t).

[0100] The below-described data separation part 133 within a mobilestation and the data separation part 127 of the terrestrial station sidesystem 102 perform the process of FIG. 7 reversely (except for theprocessing of the delay part 105) to obtain the GPS signal and thenavigation message.

[0101] Namely, this GPS information 141 is demodulated with the spreadcode held for each satellite, to extract P(t), and to obtain the GPSsignal C(t)D(t). Further, by extracting D(t) from the signal C(t)D(t) bylow frequency band-pass corresponding to D(t), the information bitsequence P(t) of the navigation message shown in FIG. 3 is obtained.

[0102] By this, a mobile station 103 does not need to consider atransmission time discrepancy owing to a distance between theterrestrial station and each satellite, which is characteristic to abroadcast GPS. Namely, since a delay corresponding to a difference in atransmission time is previously generated before transmission thatallows a time margin, it is possible to omit a discrepancy cancellingprocess after the reception that requires consideration of the real timeproperty.

[0103] (2) Quasi-zenithal Satellite (HEO Satellite)

[0104] An HEO satellite 101 comprises a satellite control part 111, afrequency modulator 112, a receiving antenna 114 for receiving GPSinformation 141 from a terrestrial station antenna 129, and atransmitting antenna 117. The satellite control part 111 controls thefrequency modulator 112 to decode the navigation message from the GPSinformation 141 received through the receiving antenna 114, and modifiesthe orbit of the HEO satellite 101 based on the decoded navigationmessage. Further, the satellite control part 111 amplifies the signal ofthe received GPS information 141, and sends the amplified signal as theGPS information 142 to the transmitting antenna 117, to transmit the GPSinformation 142 toward the ground.

[0105] (3) Mobile Station (Mobile Station Side System)

[0106] A mobile station 103 comprises a receiving antenna 136 forreceiving GPS information 141 from an HEO satellite 101, and aninformation terminal 105 with positioning function for performingpositioning calculation to obtain its own position from the GPSinformation received through the receiving antenna.

[0107] The information terminal 105 comprises: a terminal control part131 for controlling the other processing parts constituting theinformation terminal 105; a data separation part 133 for receiving GPSinformation 141 through the receiving antenna 137 to separate thenavigation message and the GPS signal from the GPS information 141; aterrestrial station position information storing part 136 for storingterrestrial station position information, obtaining it from thenavigation message separated by the data separation part 133; a positioncalculation part 132 for calculating the position from the terrestrialstation position information obtained from the GPS signal and thenavigation message separated by the data separation part 133; a map datastoring part 134 for storing 3D map data including height informationfor navigation; a clock information generating part 135 for generatingclock information expressing the present time; and the terrestrialstation position information storing part 137 for storing theterrestrial station position information.

[0108] In the following, operation of the mobile station 103 will bedescribed. A part of the operation is shown in FIG. 8.

[0109] The receiving antenna 136 receives the signal including the GPSinformation 142 from an HEO satellite 101, and delivers it to the dataseparation part 133 of the information terminal 105.

[0110] In the information terminal 105, the control part 131 makes theinformation terminal's components perform following processing.

[0111] The data separation part 133 separates the received signal toextract the GPS information 142. Further, by demodulating the GPSinformation 142 with the PN code assigned to each satellite, the dataseparation part 133 obtains the GPS signal and the navigation message(Process 801).

[0112] Further, the data separation part 133 decodes the navigationmessage (Process 802), and delivers the position information (X₁, Y₁,Z₁: i is a satellite number) of each HEO satellite 101 included in thenavigation message to the GPS position calculation part 132. Further,the data separation part 133 extracts the distance (B₁; i is a satellitenumber) between the terrestrial station side system 102 and each HEOsatellite 101 from the navigation message (or, extracts the terrestrialstation position information and position information of each satellite,and then, obtains the distance based on a difference of thosepositions), and stores the distance into the terrestrial stationposition information storing part 136 (Process 803 and Process 804).

[0113] Further, the data separation part 133 generates an event toinform the on-board control part 131 about a timing at which the dataseparation part 133 receives the top bit of the GPS signal of thepseudo-noise which is repeated at constant intervals.

[0114] The on-board control part 131 receives the time when it isinformed of the timing, from the clock information generating part 135,and set it into the GPS position calculation part 132.

[0115] The GPS position calculation part 132 calculates the presentposition of the information terminal based on the position information(X₁, Y₁, Z₁: i is a satellite number) of each HEO satellite 101 and thetiming (T₁) of receiving the signal of each HEO satellite 101 receivedfrom the data separation part 133 or the on-board control part 131(Process 805 and Process 806).

[0116] Namely, the GPS position calculation part 132 calculates adifference T₁ between a planned output timing (for example, every 10msec) of the satellite 101, which is stored in the GPS positioncalculation part 132, and the observed signal receiving timing T₁. Theposition (x₀, y₀, z₀) and the time discrepancy (_(t)) are obtained bythe following method, based on this T₁, the position information (X₁,Y₁, Z₁: i is a satellite number) of each satellite, and the distance(B₁) between each satellite 101 and the GPS reference station, which isstored in the terrestrial station information storing part.

[0117] In one time three-dimensional positioning, satellite signals arereceived from four GPS satellites, respective distances between theobservation point and positions of these four GPS satellites areobtained, and four simultaneous equations are set up to obtain asolution.

[0118] Further, in one time two-dimensional positioning, satellitesignals are received from three GPS satellites, respective distancesbetween the observation point and positions of these three GPSsatellites are obtained, three simultaneous equations are set up, oneequation relating to known values on the observation point is set up,and the four simultaneous equations consisting of those three equationsand the one equation are solved using the least-squares method.

[0119] Namely, in the three-dimensional positioning in the X-Y-Zrectangular coordinate system having the center of the earth as theorigin, expressing the position P₀ of the observation point as (x₀, y₀,z₀), the position P₁ of 1-th (i=1, 2, 3, 4) GPS satellite as (x₁, y₁,z₁), a radio wave arriving time from the i-th GPS satellite as T₁, and atime error as t, the following equation is obtained.

{(x ₁ −x ₀)²+(y ₁ −y ₀)²+(z ₁ −z ₀)²}^(½) +B ₁ =c·(T ₁+_(t))  (Eq. 1)

[0120] Defining R₁ and s as c·T₁=R₁ and c·=_(t)=s, we obtain:

{(x ₁ −x ₀)²+(y ₁ −y ₀)²+(z ₁ −z ₀)²}^(½) +B ₁ −s=R ₁  (Eq. 2)

[0121] Since we have Eq. 2 for each of the four GPS satellite, the fourunknowns x₀, y₀, z₀, and s can be obtained, which leads to a highlyprecise position x₀, y₀, z₀ of the observation point with the time errorbeing corrected.

[0122] By performing a predetermined coordinate transformation, thelongitude, latitude, and height of the observation point are obtained.Usually, however, one measurement produces an error. Accordingly, theGPS position calculation part performs observations using data of aplurality of times, and selects the solution having the least varianceamong the values as the satellite position to return to the on-boardcontrol part.

[0123] When it is impossible to see four or more satellites in theshadow of a building for example, the satellite position calculationpart performs two-dimensional observation using already-obtainedmeasurements and height information h within the map data storing part.

[0124] In the two-dimensional positioning, Eq. 1 is set up for three GPSsatellites (i=1, 2, 3). And, adding the following equation Eq. 3:

(x ₀ ² +y ₀ ² +z ₀ ²)^(½) =h  (Eq. 3)

[0125] to those three equations, the four unknowns x₀, y₀, z₀, s aresolved, which leads to a highly precise position x₀, y₀, z₀, with thetime error being corrected. By performing the predetermined coordinatetransformation, the longitude, latitude, and height of the observationpoint are obtained.

[0126] Further, the GPS position calculation part 132 refers to the mapdata using the calculated position data (x₀, y₀), compares the heightdata of that longitude and latitude with the calculated height data.When the difference between those height data is more than apredetermined value, the GPS position calculation part 132 calculatesnew height information again. This calculation is repeated until thedifference between the calculated result and the height information inthe map data is less than or equal to the predetermined value.

[0127] In the satellite NAVSTAR used in the conventional GPS system,satellites fly longitudinally and latitudinally in the sky. On the otherhand, quasi-zenithal satellites have apparently one limited orbit seenfrom the ground, and it is difficult to attain precision with a smallnumber of satellites. However, when five quasi-zenithal satellites areused as in the present invention, Geometrical Dilution Of Precision(GDOP), which also becomes a problem as an error of positioning, can bemade less than or equal to 10.

[0128] With respect to this GDOP, when four satellites are used todecide the position (x, y, z, t) of a moving object, an error generatedowing to positions of the satellites is obtained by Eq. 4.$\begin{matrix}{{A = \begin{bmatrix}x_{1} & y_{1} & z_{1} & 1 \\x_{2} & y_{2} & z_{2} & 1 \\x_{3} & y_{3} & z_{3} & 1 \\x_{4} & y_{4} & z_{4} & 1\end{bmatrix}}{{GDOP} = \sqrt{{Trace}\left( \left( {}^{i}{AA} \right)^{- 1} \right)}}} & \text{Eq.~~4}\end{matrix}$

[0129] When GDOP is calculated using another embodiment where the kindof satellites is modified from the above embodiment, the result is shownin FIG. 9.

[0130]FIG. 10 shows apparent positions of an HEO satellite 101 seen inTokyo according to each embodiment. In FIG. 10, numbers in the column ofSatellite position show positions at intervals of one hour, and thenumber 0 indicates the case where the satellite exists at the azimuth of184 degrees and the elevation angle of 0.7 degrees from Tokyo at 0000hour. For example, the number 14 with respect to the quasi-zenithalsatellite means the position at 14:00 hours.

[0131] From the result of FIG. 9, the following can be seen.

[0132] When four quasi-zenithal satellites are operated in a 6 hourcycle, GDOP becomes more than 14, and, in a certain case, a large numbersuch as 375, which is unsuitable for positioning.

[0133] On the other hand, when one satellite is added to realize afive-satellite system (in an about 5 hour cycle), the result is largelyimproved to have the worst GDOP of 9.5. Further, in a six-satellitesystem, GDOP is improved to 8.0. Further, when a geostationary satelliteof NSTAR is added to a system of four HEO satellites, GDOP becomeslargely improved to the worst of 7.6. As a result, it is found that, byadding an HEO satellite or another type of satellite to four HEOsatellites, the resultant satellite arrangement gives a GDOP valueusable for positioning. Further, even in a three-HEO-satellite system,GDOP can be improved by adding geostationary satellites. For example,when an NSTAR satellite is added, in the case of an HEO satellite orbit,GDOP can be made a value of practical use of less than or equal to 10 byarranging five or more satellites at generally uniform intervals on theorbit.

[0134] Further, other experiments show the following results.

[0135] By using three out of six quasi-zenithal satellites arranged atgenerally uniform intervals, and using, in addition, a satellite that isnot quasi-zenithal (for example, JCSAT4), GDOP can be made less than orequal to 10.

[0136] Further, using three out of four quasi-zenithal satellitesarranged at generally uniform intervals, and using, in addition, asatellite that is not quasi-zenithal (for example, JCSAT4), GDOP can bemade less than or equal to 11.

[0137] Further, using six quasi-zenithal satellite arranged at generallyuniform intervals, and using, in addition, two satellites that are notquasi-zenithal (for example, JCSAT4), GDOP can be made less than orequal to 8.

[0138] Thus, when positions of quasi-zenithal satellites are used forpositioning, it is possible to realize positioning accuracy of GDOP 10or less using a small number of satellites such as about fivesatellites.

[0139] Next, another embodiment of the present invention will bedescribed.

[0140] This system transmits GPS information using a broadcast functionof an HEO satellite, and at the same time, transmits differentialinformation from the HEO satellite. By this, one antenna can receive notonly the GPS information 141 but also DGPS information 1141 as shown inFIG. 11.

[0141] This system is characterized in that it is provided with aplurality of GPS reference station 1108 each having the same function asa mobile station 1103 and receiving GPS information.

[0142] Each reference station 1108 comprises: a receiving antenna 1105for receiving a signal that includes GPS information 142 transmittedfrom an HEO satellite 1102; and a reference station communicationapparatus 1106.

[0143] As shown in FIG. 13, this reference station communicationapparatus 1106 has a configuration similar to one of the communicationapparatus 105 of a mobile station 103 of FIG. 1 except that it does nothave the map data 134, that it is provided with a differential datagenerating part 1342 for generating differential data used forcorrecting positioning information based on information of the GPSposition calculation part, a reference information storing part 1343, atransmission part 1344 for transmitting the generated differential datato the GPS terrestrial station, and that it is provided with a referencestation control part 1341 instead of the mobile station control part131.

[0144] When the reference station 1108 receives a signal including GPSinformation 142 through the receiving antenna 1206, the data separationpart separates the signal into GPS information 142 from each HEOsatellite 101, to obtain a GPS signal.

[0145] Further, the position calculation part calculates the position ofthe reference station 1108 based on the separated GPS signal and presenttime information from the clock information generating part.

[0146] Further, the reference station control part 1341 calculates adiscrepancy from the information measured by the GPS signal, based onthe position information from the position calculation part and thereference station position in the reference information storing part1343.

[0147] The differential data generating part 1342 gives the referencestation's characteristic number to the information of the positiondiscrepancy and a time error from the position calculation part, andtransmits the resultant data as differential data to the GPS terrestrialstation through the transmission part 1344 and the network.

[0148] At that time, as the discrepancy, may be used (X, Y, Z, T) foreach satellite, or a pair of the distance difference and timediscrepancy (R, T).

[0149] As shown in FIG. 12, the terrestrial station has a configurationin which a differential data adding part 1201 is added to theterrestrial station of FIG. 1.

[0150] The differential data adding part 1201 receives differential datafrom a plurality of reference stations 1108, and transmits the receiveddata as DGPS information to the HEO satellite 101.

[0151] Receiving the DGPS information, the HEO satellite 101 transmitsit at a predetermined frequency toward the ground through thetransmitting antenna, after frequency modulation if necessary.

[0152] As shown in FIG. 14, the mobile station 1103 is newly added witha DGPS correction calculation part 1401 and an adjacent terrestrialstation judgment part 1402, in comparison with the mobile station ofFIG. 1.

[0153] In the present embodiment, when the DGPS information 1142, whichhas been modulated with a predetermined frequency and a spread code, isreceived through the antenna receiving part 136, the data separationpart separates the received information.

[0154] The on-board control part obtains the DGPS information 1142 ofthe nearest GPS reference station, using the reference station positioninformation in the separated DGPS information 1142 and firstapproximation position obtained from the GPS position calculation part,and sets the obtained DGPS information 1142 to the DGPS correctioncalculation part 1401.

[0155] Further, the position calculation part calculates its ownposition again.

[0156] At this time, however, the calculation is performed as in Eq. 5using discrepancy information for each satellite.

{(x ₁ −x ₀)²+(y ₁ −y ₀)²+(z ₁ −z ₀)²}^(½) +B ₁ =c·(T ₁ −T+ _(t))−R  (Eq.5)

[0157] where (R, T) is the discrepancy information of the satelliteconcerned.

[0158] Namely, the GPS position calculation part refers to the DGPSinformation of the nearest GPS reference station in the DGPS correctioncalculation part 1401, and subtracts the discrepancy measured by thereference station from each position data calculated, to obtain the realposition.

[0159]FIG. 15 shows the GPS information generating part according to thepresent embodiment.

[0160] In this example, the DGPS information shown in FIG. 16 is addedafter the navigation message and transmitted as the data in the GPSinformation.

[0161] As shown in FIG. 16, DGPS information has a message header forsynchronization similar to the navigation message, a message type (inthis case, 2 indicating DGPS information) showing a kind of the message,and thereunder, the number of the reference stations, and for eachreference station, a reference station identification number, referencestation position information, the number of satellites seen from thereference station, and for each satellite, a satellite number, acorrection value for a pseudo distance, a correction value for adistance change rate, and a data issue number indicating datameasurement time. The mobile station distinguishes between a navigationmessage and DGPS information, based on the message type shown in FIG.16.

[0162] Conventionally, the differential GPS has been realized using aplurality of media or antennas. Using the present system, however, it ispossible to establish a three-dimensional positioning system in whichone antenna receives GPS positioning information and differentialinformation for making the GPS positioning information more precise.

[0163] Here, will be described a system for communication with acommunication business, using the positioning system of the presentinvention

[0164]FIG. 17 shows a system configuration of the present embodiment.

[0165] The system of the present embodiment comprises fivequasi-zenithal satellites (hereinafter, referred to as HEO satellites)101, terrestrial station antennas 129, a terrestrial station sidecommunication system 1700, a mobile station side communication apparatus1709, an access point 1703, and a telecommunication business side system1704.

[0166] An HEO satellite 101 has the same configuration as the HEOsatellite 101 of FIG. 1.

[0167] The terrestrial station side system 1700 has a configurationgenerally similar to the communication apparatus 104 of FIG. 1, exceptthat a communication apparatus 1702 for controlling a communication linewith the access point 1703 is provided. Further, the access point 1703is connected to the communication apparatus 1705 of thetelecommunication business 1704 that performs Internet connection andthe like. Further, it is assumed that a plurality of such terrestrialside systems 102 are provided.

[0168] Further, the mobile station side communication apparatus 1709 hasa configuration that is obtained by adding a terrestrial stationselecting part 1706, a terminal control part 1707, and a communicationcapacity storing part 1708 to the configuration of the mobile stationside communication apparatus 105 of FIG. 1.

[0169] First, a user makes input specifying a communication businessinto the mobile station side communication apparatus 1709 as aninformation terminal of the mobile station side system 103. When theinformation terminal receives that input, searches and specifies theterrestrial station side system whose line is connected to the specifiedcommunication business (Process 1801).

[0170] The position of the terrestrial station side system used forpositioning is obtained from the terrestrial station positioninformation storing part 137 (Process 1802).

[0171] Communication capacity between the communication business 1704 orthe access point 1703 and the terrestrial station side communicationsystem 1700 is obtained from the communication capacity storing part1708 (Process 1803).

[0172] Next, the terrestrial station selecting part 1706 obtains thecommunication capacity of the communication line between thecommunication business 1704 or the access point 1703 and eachterrestrial station 101, and selects the terrestrial station side systemconnected to the communication line having the largest communicationcapacity (Process 1804).

[0173] By selecting the terrestrial station side system to connect,using the positions of the terrestrial station side systems used forpositioning, it is possible to select the communication line having thelargest communication capacity between the communication business 1704or the access point 1703 and the terrestrial station 101. Accordingly,it is possible to shorten the communication time between the informationterminal and the communication business, and improvement of thecommunication speed. Also, from the viewpoint of the user of theinformation terminal, a communication charge can be reduced byshortening the communication time.

[0174] While we have shown and described the embodiments according toour invention, it should be understood that the disclosed embodimentsare susceptible of changes and modifications without departing from thescope of the invention. Therefore, we do not intend to be bound by thedetails shown and described herein, but intend to cover all such changesand modifications falling within the ambit of the appended claims.

What is claimed is:
 1. A positioning system comprising: a terrestrialstation side system, which comprises a terrestrial station sidesatellite communication antenna for transmitting a signal to a satelliteand a terrestrial station side communication apparatus for transmittingthe signal to said terrestrial station side satellite communicationantenna; said satellite having a satellite side satellite communicationantenna for performing transmission and reception to and from a ground;and a mobile station side system, which comprises a mobile station sidesatellite communication antenna for receiving said signal from thesatellite, and an information terminal for measuring a position wherethe mobile station side system exists, based on the signal receivedthrough said mobile station side satellite communication antenna;wherein: said information terminal comprises: a means for extracting adistance between the position where said terminal station side systemexists and said satellite, from the signal transmitted from saidsatellite; a means for extracting a transmission time from said signaltransmitted from said satellite; a means for obtaining a sum of thedistance between said terrestrial station side system and the satelliteand a distance between said satellite and the mobile station sidesystem, based on a difference between said transmission time extractedand a reception time of said signal in said mobile station side system;a means for obtaining the distance between said mobile station sidesystem and said satellite, by subtracting the distance between saidterrestrial station side system and said satellite from said sum; and ameans for obtaining the position where said mobile station side systemexists, based on the obtained distance between said mobile station sidesystem and said satellite.
 2. The positioning system according to claim1, wherein: the signal transmitted from said satellite includes anavigation message; and said navigation message includes the informationindicating position where said terrestrial station side system existsand a position of said satellite.
 3. The positioning system according toclaim 1, comprising: three or more quasi-zenithal satellites as saidsatellite.
 4. The positioning system according to claim 2, comprising:three or more quasi-zenithal satellites as said satellite.
 5. Thepositioning system according to claim 3, wherein: a number of saidquasi-zenithal satellites included in the positioning system is four ormore.
 6. The positioning system according to claim 4, wherein: a numberof said quasi-zenithal satellites included in the positioning system isfour or more.
 7. The positioning system according to claim 5, wherein: anumber of said quasi-zenithal satellites included in the positioningsystem is five or more.
 8. The positioning system according to claim 6,wherein: a number of said quasi-zenithal satellites included in thepositioning system is five or more.
 9. The positioning system accordingto claim 3, wherein: each of said quasi-zenithal satellites is an HEOsatellite.
 10. The positioning system according to claim 4, wherein:each of said quasi-zenithal satellites is an HEO satellite.
 11. Thepositioning system according to claim 5, wherein: each of saidquasi-zenithal satellites is an HEO satellite.
 12. The positioningsystem according to claim 6, wherein: each of said quasi-zenithalsatellites is an HEO satellite.
 13. The positioning system according toclaim 7, wherein: each of said quasi-zenithal satellites is an HEOsatellite.
 14. The positioning system according to claim 8, wherein:each of said quasi-zenithal satellites is an HEO satellite.
 15. Aninformation terminal, comprising: a means for extracting a distancebetween a terrestrial station side system and a satellite, from a signaltransmitted from said satellite; a means for extracting a transmissiontime from said signal transmitted from said satellite; a means forobtaining a sum of the distance between said terrestrial station sidesystem and said satellite and a distance between said satellite and amobile station side system, based on a difference between saidtransmission time extracted and a reception time of said signal receivedby said mobile station side system; a means for obtaining the distancebetween said mobile station side system and said satellite, bysubtracting the distance between said terrestrial station side systemand said satellite from said sum; and a means for obtaining a positionwhere said mobile station side system exists, based on the obtaineddistance between said mobile station side system and said satellite. 16.The information terminal according to claim 15, wherein: said distancebetween said terrestrial station side system and said satellite isincluded in a navigation message included in said signal.
 17. Theinformation terminal according to claim 16, wherein: said satellite iseach of three or more quasi-zenithal satellites.
 18. The informationterminal according to claim 17, wherein: a number of said quasi-zenithalsatellites is four or more.
 19. The information terminal according toclaim 18, wherein: a number of said quasi-zenithal satellites is five ormore.
 20. The information terminal according to claim 17, wherein: eachof said quasi-zenithal satellites is an HEO satellite.
 21. Theinformation terminal according to claim 18, wherein: each of saidquasi-zenithal satellites is an HEO satellite.
 22. The informationterminal according to claim 19, wherein: each of said quasi-zenithalsatellites is an HEO satellite.
 23. A method of positioning comprisingsteps of: extracting a distance between a terrestrial station sidesystem and a satellite, from a signal transmitted from said satellite;extracting a transmission time from said signal transmitted from saidsatellite; obtaining a sum of the distance between said terrestrialstation side system and the satellite and a distance between saidsatellite and a mobile station side system, based on a differencebetween said transmission time extracted and a reception time of saidsignal received by said mobile station side system; obtaining thedistance between said mobile station side system and said satellite, bysubtracting the distance between said terrestrial station side systemand said satellite from said sum; and obtaining a position where saidmobile station side system exists, based on the obtained distancebetween said mobile station side system and the satellite.
 24. Acomputer readable storage medium holding a program to be executed by acomputer to perform a positioning method, said positioning methodcomprising steps of: extracting a distance between a terrestrial stationside system and a satellite, from a signal transmitted from saidsatellite; extracting a transmission time from said signal transmittedfrom said satellite; obtaining a sum of the distance between saidterrestrial station side system and the satellite and a distance betweensaid satellite and a mobile station side system, based on a differencebetween said transmission time extracted and a reception time of saidsignal received by said mobile station side system; obtaining thedistance between said mobile station side system and said satellite, bysubtracting the distance between said terrestrial station side systemand said satellite from said sum; and obtaining a position where saidmobile station side system exists, based on the obtained distancebetween said mobile station side system and the satellite.
 25. Acomputer program product comprising: a computer readable program codemeans for extracting a distance between a terrestrial station sidesystem and a satellite, from a signal transmitted from said satellite; acomputer readable program code means for extracting a transmission timefrom said signal transmitted from said satellite; a computer readableprogram code means for obtaining a sum of the distance between saidterrestrial station side system and the satellite and a distance betweensaid satellite and a mobile station side system, based on a differencebetween said transmission time extracted and a reception time of saidsignal received by said mobile station side system; a computer readableprogram code means for obtaining the distance between said mobilestation side system and said satellite, by subtracting the distancebetween said terrestrial station side system and said satellite fromsaid sum; and a computer readable program code means for obtaining aposition where said mobile station side system exists, based on theobtained distance between said mobile station side system and thesatellite.